Various auxiliary noise sources exist in aircraft. One specific source of noise in aircraft equipped with gas turbine engines is caused by the flow of fan bypass air across the entrances of dead headed conduits leading to the engine fan bleed air system. Fan bypass air is air driven by fan and diverted around the gas generator to provide a significant portion of the engine thrust. Fan bleed air is air that is diverted away from the bypass air for auxiliary uses, such as cooling.
The fan bleed air system provides cooling air from the fan bypass air plenum to various parts of the aircraft via a heat exchanger that is often mounted in the engine pylori that secures the engine to the fuselage of the aircraft. When bleed air is not required, a fan air valve mounted in a cooling conduit upstream of the heat exchanger is closed. This creates a Helmholtz Resonator from the stub portion of the conduit located between the fan air valve and the open conduit bellmouth.
The flow of fan bypass air past the entrance creates an area of low pressure at the conduit entrance causing some air to enter the conduit. As air enters the conduit, a pressure buildup occurs in the conduit against the closed fan air valve. When the pressure builds to a certain peak level, the high pressure air in the dead headed conduit reverses direction and exits the conduit resulting in a relatively low pressure level in the conduit due to the inertia of the air exiting the conduit. The repetitive process of pressurizing and depressurizing the dead headed conduit causes a harmonic reverberation similar to that made by a bottle as one blows across the bottle opening. The exact frequency and intensity of the sound will depend on the speed of the fan bypass air through the engine and primarily, the length of the fan bleed air conduit. As a non-limiting example, a model AS907-3-1E gas turbine engine bleed air dead headed conduit produces sound in the 400 Hz range.
Because the generation of noise can create an undesirable nuisance, it is desirable to eliminate the unnecessary noise stemming from the fan bypass air system. In addition, it is desirable to do so with the least number of moving parts which keeps maintenance costs low and reliability high. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.